1. Field Of The Invention
The present invention relates to a device for overvoltage protection of a rectifier bridge feeding a d.c. motor and implemented using fully gate-controlled solid-state switches without zero diodes, and for control of the d.c. motor during emergency braking.
2. Description Of Related Prior Art
In current practice, overvoltage protection of rectifier bridges and control of d.c. motors during emergency braking are implemented separately. Especially when an elevator has to be stopped in an emergency, the braking must be performed as independently as possible, without regard to the rest of the elevator system. Often official safety regulations prescribe the use of electric braking. Electric braking is currently achieved by connecting a braking resistor to the motor terminals either by means of an electromechanical switch or thyristors. One solution is proposed in FI patent publication 76226. The braking torque is proportional to the current flowing in the circuit, i.e. it depends on the terminal voltage of the motor and the total resistance of the braking circuit. In other cases, the braking is effected using a mechanical brake only.